This is a four-arm, randomzied, partially-blind, multicenter, Phase II study of nucleoside analogues given alone or in combination to subjects with CD4+ cell counts between 300-600/mm3 and greater that 24 weeks of ZDV experience. The four treatment arms are 1) d4T alone, 2) ddI alone, 3) ZDV and ddI in combination, 4) ZDV and d4T in combination. The administration of d4T and ddI will be open-label. The study is double- blind with respect to ZDV treatment. The duration of the study is 48 weeks after the enrollment of the last subject. A 12 week blinded study extension wil be offered to all subjects still on blinded therapy upon the conclusion of the original 48 weeks after the last subject is enrolled.